Surface functionalized biochar for the effective capacitive deionization of lead ions

Biochar has emerged as a promising alternative to activated carbon for capacitive deionization (CDI) due to its special architecture and functionalities. However, the selective purification of trace toxic ions is always limited by the number of specific functionalities. Herein, a thiosemicarbazide-modified biochar (BC-S) was developed for removing trace Pb2+ from water. The cotton stalk derived biochar (BC) is prepared by one-step activation with CO2, which has a high mesoporous ratio and is favorable for ion transport. It was then chemically modified with thiosemicarbazide to add additional Pb2+ adsorption sites. BC as anode and BC-S as cathode formed an asymmetric device for testing, and it exhibited a Pb2+ adsorption capacity of 68.5 mg g−1, much higher than that of the symmetric BC electrode (15.9 mg g−1). More importantly, BC-S still performs excellent Pb2+ selectivity in the presence of various cations such as Ca2+ and Mg2+. The ex-situ XPS and SEM revealed that Pb2+ is adsorbed by −C-S, −C = S, −SOx-, and −N- functional groups on the electrode surface, where electrosorption and chemisorption co-occur. This work shows that biochar can be engineered and explored broadly as an inexpensive sustainable electrode material for the separation and recovery of heavy metals and salt ions from water